This invention relates generally to means for protecting thyristor power conversion systems and more particularly to a coordinated protection system for a load commutated inverter motor drive including a source side converter and a load side converter coupled together via a DC link circuit.
Many circuits and systems are known for controlling the conductivity of controlled rectifiers utilized in various types of power converters for supplying electrical power to a load such as an AC motor from a polyphase alternating current (AC) source. The type of rectifier used controls, to some degree, the type of control utilized but by far the most common controlled rectifier used in practice today is a thyristor of the silicon controlled rectifier (SCR) type which becomes conductive with the simultaneous application of a forward bias voltage and a signal applied to its gate electrode and which thereafter remains conductive until the anode current falls below the value required to hold the thyristor in its conductive state. Furthermore, motor control systems are known which utilize thyristor circuits in various configurations via a DC link circuit.
Whereas analog control techniques, a typical example being U.S. Pat. No. 4,230,979 entitled, "Control Current Inverter And Motor Control System", issued to Paul M. Espelage, et al. on Oct. 28, 1980, were first employed in such motor control systems, attention now is being directed to digital types of control systems. Examples of digital type systems are disclosed in: U.S. Pat. No. 3,601,674, entitled, "Control System For Firing SCR's In Power Conversion Apparatus", which issued to John A. Joslyn, et al. on Aug. 24, 1971; U.S. Pat. No. 4,263,557, entitled, "Power Converter Control", which issued to Willard B. Jarvinen on Apr. 21, 1981; and U.S. Pat. No. 4,276,505, entitled, "Microcomputer Based Control Apparatus For A Load Commutated Inverter Synchronous Machine Drive System", which issued to Bimal K. Bose on June 30, 1981. The teachings of these patents are also intended to be incorporated by reference. Additionally in the aforementioned related applications, there are disclosed further improvements in digitally controlled load commutated inverter motor drive systems. Such systems, moreover, include control means which are implemented by the software included in microprocessor apparatus.
It is also well known that thyristor power conversion systems include means which are adapted to protect the system against one or more fault conditions by sensing the occurrence of a particular type of fault and thereafter causing some corrective action to be taken to prevent consequential damage, primarily to the thyristors. In some instances the system is adapted to recover from momentary faults, but in the event of hard faults or conditions indicative of a hard fault, the system is shut down in an orderly fashion. Typically, the type of faults detected include undervoltage and overvoltage, overcurrent, reverse current, shoot through, commutation failure and ground faults. In the case of motor controls, motor overspeed is also protected against.
Circuitry for monitoring and sensing the state of the thyristor cells of a power converter are shown, for example, in U.S. Pat. No. 3,654,541, entitled, "Thyristor State Sensor", issued to F. W. Kelly, et al. on Apr. 4, 1972 and U.S. Pat. No. 3,806,906, entitled, "Cell Surveillance Monitor For A Power Converter", issued to J. A. Young on Apr. 23, 1974.
Typical examples of protective circuitry for overcurrent and overvoltage conditions are disclosed in: U.S. Pat. No. 3,413,538, entitled, "Control Means For Starting Electric Power Converters At Reduced Operating Frequencies", issued to M. Hodges on Nov. 26, 1968; U.S. Pat. No. 3,750,004, entitled, "Instantaneous Current Control For Static Inverters", issued to L. Walker on July 31, 1973; U.S. Pat. No. 4,237,531, entitled, "Controlled Current Inverter System Having Semiconductor Overvoltage Protection", issued to J. H. Cutler, et al. on Dec. 2, 1980, and U.S. Pat. No. 4,272,816, entitled, "Overcurrent Protecting Apparatus", issued to Y. Matsumoto on June 9, 1981.
Examples of methods and apparatus for detecting and correcting for shoot through and commutation faults are disclosed in: U.S. Pat. No. 4,016,468, entitled, "Controlled Rectifier Motor Drive System Including DC Fault Detection And Commutation Means", issued to C. Graf on Apr. 5, 1977; U.S. Pat. No. 4,107,771, entitled, "Circuit For Shuting Down An Inverter", issued to T. Anderson, et al. on Aug. 15, 1978; U.S. Pat. No. 4,150,325, entitled, "Power Conversion System Having Conversion Fault Detection And Correction Circuit", issued to R. Miller, et al. on Apr. 17, 1979; U.S. Pat. No. 4,183,081, entitled, "Inverter System Including Commutation Failure Prevention Scheme", issued to J. Cutler, et al. on Jan. 8, 1980; U.S. Pat. No. 4,197,575, entitled, "Conduction Through Sensing And Inverter Protection System", issued to J. Young, on Apr. 8, 1980; U.S. Pat. No. 4,218,729, entitled, "Polyphase Rectifying Bridge With Commutation Fault Protection", issued to G. Chambers on Aug. 19, 1980, and U.S. Pat. No. 4,270,078, entitled, "Method And Apparatus For A Variable Frequency Inverter System Having Commutation Fault Detection And Correction Capabilities" issued to L. Walker, et al. on May 26, 1981.